Binders which have been commonly used for magnetic recording media include, for example, vinyl chloride-vinyl acetate resins, alone or as mixtures thereof with one or more polymers selected from cellulose derivatives, acrylic resin (i.e., copolymers of acrylic acid or methacrylic acid and esters thereof), urethane resins, vinylidene chloride copolymers, synthetic rubbers, polyesters, etc., and reactive mixtures comprising the above-described mixture and a polyisocyanate, which are cured by crosslinking.
However, when these conventional binders are used in video tapes, the vinyl chloride-vinyl acetate resin contained in video tapes forms and releases hydrochloric acid, which ultimately causes corrosion of metals of the running system, e.g., guide poles, or a magnetic head of a video deck or ferromagnetic metal particles.
On the other hand, binders not containing vinyl chloride-vinyl acetate resins have a disadvantage of relatively high cost.
In magnetic recording tapes for audio or video devices or computers, the surface of a magnetic recording layer generally has a smooth finish in order to ensure good sensitivity, and, in particular, good output power in the high frequency region. However, such a smooth magnetic recording tape is liable to disorder in winding and rewinding. Use of such a magnetic recording tape causes deterioration of running properties and variation in output and also tends to induce deformation or damage of the tape.
In order to overcome this problem, it has been proposed to provide a backing layer on the surface of a support opposite to the magnetic recording layer. Binders which have conventionally been used for the backing layer include vinyl chloride-vinyl acetate copolymers and the like. Backing layers using these conventional binders, however, adversely affect running durability of the tape due to their poor wear resistance, toughness, etc., thus resulting in an increased coefficient of friction, folding of the tape, and the like. Therefore, improvements in the backing layer has been strongly desired.
In addition, when a backing layer is provided generally for the purpose of improving running characteristics or running durability, it is known that when a magnetic medium is rolled up (particularly if being in a tape shape) or piled up (if being in a sheet shape), surface roughness of the backing layer is transferred to the surface of the magnetic layer to impair surface properties of the magnetic layer, which leads to deterioration in electromagnetic properties of the magnetic medium, especially variation in output power. This is because the surface of the backing layer is made too much rough from the standpoint of running properties, and such a surface roughness is transferred to the magnetic layer surface when the magnetic medium is preserved in storage or allowed to stand in roll form or piles.
As described above, although use of a mixture of a vinyl chloride-vinyl acetate resin as a main binder and other resins as a binder for magnetic recording media, such as video tapes, is economically advantageous, not only such a binder corrodes the running system of a video deck, a permalloy head, or a metal magnetic material, but also the dispersibility of fine ferromagnetic particles in such binder is too poor to ensure high chroma S/N ratios. Moreover, the above-described binder cannot sufficiently satisfy other requirements for video tapes, such as running properties, still life, and the like.
In order to overcome these disadvantages, it is described, e.g., in Japanese Patent Application (OPI) No. 200426/83 (the term "OPI" as herein used means "unexamined published application"), to use a phenoxy resin, a polyurethane elastomer, and a polyisocyanate as a binder. This phenoxy resin, however, has a difficulty in terms of solubility, and therefore it cannot be applied to use with ease and convenience.